1. Field of the Invention
This invention is concerned with dewaxing a middle distillate to reduce the pour point thereof. It is particularly concerned with catalytically dewaxing a petroleum distillate utilizing as dewaxing catalyst a crystalline zeolite having a Constraint Index of 1 to 12 as further defined below, and a silica to alumina ratio of at least 12. It is still more particularly concerned with such dewaxing process wherein the olefinic by-product formed in the dewaxing step is treated separately with a catalyst such as ZSM-5 to produce an improved set of products, including a higher yield of fuel oil, all as more fully described herein below.
2. Prior Art
Catalytic dewaxing per se is known and described in U.S. reissue patent, U.S. Pat. No. Re. 28,398 to Chen et al.
The broad concept of contacting an olefinic charge with the special type of zeolite with which this invention is concerned is known in the art and is the subject of various United States patents. Thus, for example, U.S. Pat. No. 3,960,978 teaches conversion of olefins to olefinic gasolines. U.S. Pat. No. 4,021,502 discloses conversion of olefins over ZSM-12. U.S. Pat. No. 3,750,024 discloses contacting olefins with ZSM-5 type zeolites. U.S. Pat. No. 3,778,501 discloses preparation of aromatics by contacting olefins over ZSM-5 type catalysts. U.S. Pat. No. 3,827,968 discloses a two-step aromatization wherein in the first step an olefin is contacted over a ZSM-5 type zeolite. U.S. Pat. No. 3,845,150 discloses contacting a feed comprising saturated and unsaturated compounds over a ZSM-5 type zeolite in order to form aromatics. U.S. Pat. No. 3,843,740 also discloses an aromatization process wherein various feed streams, including gasoline, are contacted with ZSM-5 type zeolites. U.S. Pat. No. 3,756,942 discloses an aromatization process involving contact over ZSM-5 type catalysts of various feeds, including gasoline.
More recent patents in the same general area include U.S. Pat. No. 4,254,295 to Tabak, which describes the use of ZSM-12 in the liquid-phase oligomerization of straight and branched chain olefins which may have from 2 to 12 carbon atoms, but preferably from 2 to 6 carbon atoms; U.S. Pat. No. 4,227,992 to Garwood et al. which describes the separation of ethylene in admixture with light olefins by contacting the mixture with a member of the special class of zeolites used in the present invention. U.S. Pat. No. 4,211,640 to Garwood discloses that the gum stability property of a highly olefinic gasoline is improved by contact with the forementioned type catalyst under reaction conditions carefully selected so as to substantially avoid formation of either aromatic hydrocarbons or products lighter than C.sub.6. Some fuel oil is formed in this process.
The prior art dewaxing process wherein the catalyst used is of the ZSM-5 type is believed to operate at least in part by selectively cracking the waxy normal and singly methyl-substituted paraffins to form lower molecular weight of olefins and paraffins. Regardless of mechanism, such dewaxing results in the formation of a by-product hydrocarbon fraction that has a lower average molecular weight than the charge and a substantial content of olefins. A very considerable portion of this by-product fraction is a liquid that boils within the boiling range of gasoline, but this liquid is of limited value as gasoline because of its high mercaptan content and high bromine number. Also, in many cases fuel oil is as valuable as gasoline, or even more valuable.